A vehicle which breaks the laws of physics...

[daleyd]

I made it 1.5kmh (but I was assuming that the prop was 4m diameter)

Quite - but not near Mach 1. Even The fastest propellor planes tips don’t exceed Mach 1 - if I remember correctly there is some phenomenon which means they end up lunching themselves when they get near it…as some of the early spitfire pilots found out to their cost.

 

[Seadog]

Didn't Westland come up with an innovative rotor tip design because they were having issues with approaching mach speeds?

 

[Screwdriver]

0.468 mph?

oh bugger. What an idiot! I screwed up my radians. Hang on. Let me get my thinking head on.

<edit> I multiplied 2 rpm by 60 instead of dividing...

 

[daleyd]

diameter x pi x rpm gives it you in m/min

 

[daleyd]

Didn't Westland come up with an innovative rotor tip design because they were having issues with approaching mach speeds?

I think so, the later lynx blades were much different to the early ones, not sure if this was related to that?

 

[Screwdriver]

oh bugger. What an idiot! I screwed up my radians. Hang on. Let me get my thinking head on.

<edit> I multiplied 2 rpm by 60 instead of dividing...

More like 1600 rpm for a 2m blade but frankly, I don't trust me either.

<edit> 2m prop is a 12.56m circle. Speed of sound 343 m. 343/12.56 = 27.3 revolutions required per second. This time I should multiply by 60 to get rpm. 1638.53 rpm.

Idiot!!

 

[Seadog]

Ah, BERP III

BERP rotor - Wikipedia

en.wikipedia.org

 

[Screwdriver]

Ah, BERP III

BERP rotor - Wikipedia

en.wikipedia.org

I seem to recall there also being an issue with giant wind turbines. Apparently they regularly hit mach 0.2. Some good videos out there of them flying apart in strong winds. I shall of course resist the temptation to calculate the loads on a prop tip...

 

[spork]

The airflow is "identical" to that of an aircraft? Surely then that does imply a high pressure region is generated behind the prop? The aircraft is swapping some petrol in exchange for being able to shove some wind behind it. With the Blackbird, ignoring where the energy is coming from and imagining it's "up there" flying along, disobeying the laws of physics: it must surely also be creating a large volume (or "cushion") of disturbed air in its wake?

Alrighty, I'll try my best here...

I'm not sure what you mean when you say "disobeying the laws of physics". This thing is definitely not disobeying any laws of physics, creating or exposing any new ones, or even doing anything particularly novel. Perhaps you were just making a joke about spinning the propeller without specifying where that energy comes from. If so - great...

So yes, the propeller does generate a low pressure region in front and a high pressure region behind. And we can integrate that differential to compute the thrust generated. OR we can look at the rate of momentum change. In other words we can simply look at how much air mass is being thrown backward at what rate. But, just like a Cessna flying straight and level, we don't have to look at that increased pressure region interacting with a tailwind. When we're going downwind faster than the wind, this propeller doesn't know anything about a tailwind any more than a Cessna in flight does (I assume you're aware that an aircraft in flight has no way of knowing anything about the wind except in reference to the ground... if your GPS says you're going 200 kts while your airspeed indicator says you're going 100 kts, you have a 100 kt tailwind).

Where the wind DOES come into play has more to do with the lever analogy I mentioned. The power gained or spent = force x velocity. Because there's a tailwind, I'm moving through the air more slowly than I'm moving over the ground. So let's imagine I'm going 20 kts in a 10 kt tailwind. I'm going downwind at 2X wind speed. This means I'm going over the ground at 20 kts, and only moving through the air at 10 kts. So let's imagine the wheels feel a retarding force on the ground of... let me change to metric...

Let's say I'm going directly downwind at 20 m/s in a 10 m/s tailwind. So I'm going 20 m/s over the ground but only 10 m/s through the air. And let's assume the wheels feel a retarding force of 500 newtons. So I'm "harvesting" 10,000 newton-meters/sec by the wheels. For now let's assume no losses (i.e. no rolling resistance, aero drag, transmission losses, propeller inefficiency, etc.). We'll come back and deal with them.

So I've now got 10,000 n-m/s to play with. I'll put all of that into the propeller. It's moving through the air at 10 m/s, and I want it to generate that same 500 newtons so I can maintain my steady-state speed. So I need 10 m/s * 500 newtons (or 5000 n-m/s) to do that. Bingo - that leaves me with 5,000 n-m/s to spare. I can use that to cover rolling resistance, propeller inefficiency, aero drag, and transmission losses. In other words, I can go 2X windspeed directly downwind all day long with a vehicle that has an overall efficiency of only 50%. And the reason I can do this is that the vehicle acts just like a lever between the ground and air. I trade off a smaller force over a larger distance (the wheels rolling over the ground) for larger force over a smaller distance (the propeller moving through the air).

Back to reality for a moment and we're on the ground at 1.1x wind speed. Now we take the above scenario where the prop is generating thrust "just like" your Cessna but this time within the envelope of the prevailing wind which is still pushing it along as a bluff body.

If I'm going downwind at 1.1X wind speed, the wind can't push me as a bluff body. I'll be experiencing a headwind of 0.1X wind speed.

My current understanding/imagination tells me the wind from behind can't even reach the huge swept area of the prop because the prop is travelling 1.1x faster.

That's correct. And this is exactly why the flow through and around the propeller is exactly like that of a Cessna in straight and level flight. Keep in mind that the propeller knows nothing of what the ground is doing. It's just cruising along at 11 m/s in a 10 m/s tailwind. So all it knows is that it's operating in a 1 m/s headwind - nothing more.

Therefore in my imagination, the prevailing wind is pushing on a cushion of air left behind in the wake of the spinning prop. The vehicle is being carried along in a protective bubble of air of its own making.

Yes - this is the very common misconception I mentioned. But that bubble would have to be growing very rapidly for that tailwind to be able to continue pushing on it since we're going 10% faster than the wind. The reality is that the propeller simply does what a propeller will do when moving through the air at 1 m/s. Makes sense?

I love your description of "leverage", that really helps me to understand the dynamics. I think the tricky part which most people struggle with is the concept of relativity (no not that sort!).

You couldn't be more right! I have found it astonishing how many physics and engineering professors have argued that operating this thing on a treadmill is not equivalent to operating on the playa in a tailwind. That's day-one stuff in intro to high school physics.

It is as you say, the frame of reference which is important here. I imagine for example, a tree on its side falling out of the sky. Root and branch looks for all the world like a dumbell falling perfectly level. Now change your frame of reference and imagine that same tree travelling along sideways (for some unspecified reason) at the same rate it had been "falling".

It will carry on in this manner, zooming along through still air, generating a huge wake as it is inexplicably pushed along. Now lets introduce the ground: the roots drag on the ground and the tree suddenly tips forwards (there's your "leverage" if I understand you correctly). Replacing your Blackbird with a tree on wheels so that it can be blown along, there is still appreciably more drag wanting to tip it over forwards. Instead though, we have put our tree via a clever support on a giant skateboard which diverts this drag induced turning moment into a spin (you can see where I am going with this) - gear it up and carve the tree into a propeller etc.

Yes - that's one way to look at the leverage. Let me re-cast that slightly...

Imagine a pole that's simply balanced on the ground (standing vertically). Now let's put a big sail half way up that pole. The wind would push that sail at wind speed (almost). That means the top of the pole will be going 2X wind speed. So let's make that pole one of the spokes of a large wheel. The hub is of course at the top of that spoke. So all we need is a big wheel with a fan-fold sail half way along each spoke. We deploy that sail when the spoke is on bottom and retract it for the rest of the rotation. Voila - we have a vehicle that goes downwind at nearly 2X wind speed.

My frame of reference is the wind (that should be a sig. ) and I am standing still in a still wind while the planet rushes past my feet. The wind as a sentient being, sees the ground as an inconvenience as it is zooming along. In its frame of reference, the ground is "getting in the way" causing drag when suddenly it bumps into a tree on a skateboard and in that section of otherwise rough drag inducing groundplane, it can slide forwards a LOT more easily remembering that if a root touches the ground, it suddenly wants to lurch forwards. Little does it know some clever human has stuck a mechanism inside to nick some of that power otherwise lost to drag and instead turn it into a spinning prop.

You, my friend, understand Galilean invariance better than a disturbing number of physics professors! or (?)

 

[spork]

Sorry for the delayed response. I knew there was gonna be a lot of typing on that one, and I had some stuff going on. Now I'll try and get to some of the other questions...

 

[spork]

What would be interesting is to discover which physical barrier is the major hurdle to overcome in optimising a practical design. It might well be aero drag for all I know but I rather suspect it will be massively affected by more mundane mechanical issues; friction and drag from the gearing (obviously), wheel scrub and of course the structural integrity of a lightweight propeller spinning at potentially huge velocities.

For the Blackbird I would say the thing that got most of our focus was the propeller. It's tough to make a propeller that's more than about 85% efficient, and of course you'll lose some efficiency with a variable pitch prop anywhere off its nominal design pitch. Rolling resistance only costs you about 1% of the vehicle weight, and a chain drive can get you something like 98% efficiency. Aero drag is a thing, but not a big thing. Remember we're moving through the air a lot more slowly than we're moving over the ground. A bit of fairing, and that's not a big problem.

For example, @spork casually mentions reaching mach speed at the tips of the "propeller". That is a lot "easier" to get to than one might imagine with a large prop being blown around by wind. You only need to spin a 2m long prop at just over 2 rpm before you reach mach 1 at the tip!

Nah - I see that you guys already caught that problem. But keep in mind that those big long helicopter blades want to keep their tips below Mach 1.0. We start thinking about compressibility effects at around Mach 0.3. Also keep in mind that we didn't have to get too insane to beat the world record since there was no world record. As a result, it really won't be terribly hard for someone to break our record - and I hereby volunteer to help anyone that wants to give it a go.

So as the featherweight craft...

If only. The cart came out around 450 lbs and my fat butt adds another 200 lbs.

...is being blown along spinning like crazy, threatening to shake itself to pieces...

That's another unfortunate thing about the Veritasium video. I sold the Blackbird about 10 yrs ago since I had nowhere to keep it. The new owner has had it out several times, and the prop blade pitch has become unmatched. I wanted to fix it that day, but he was hesitant to make changes with Derek out there ready to film. So we went with it as is and it tried to shake itself apart. I've had that cart over 50 mph in the past with no shaking at all.

...with the prop struggling to hang onto it's physical integrity - it runs slap bang into a brick wall of sound. When I first saw the Veritasium video where our hero Derek "risked his life", I wonder if he really knew how close he was to the truth!

He very much did. Those pylons fold down for transport and the prop spinner ends up right where the pilot's head is. Lots of people commenting on the video said "clickbait! How's he risking his life at 10mph?".

 

[spork]

I imagine the gearbox/diff and shafts have quite some force through them given they're almost levering off each other.

We don't have a differential, but we do have ratchets on both sides of the axle. We initially sheared a pin on the prop shaft, so we made a bigger hole and used a fatter pin (that's what they taught me in engineering school! )

We also turned the upper-gear wheel into a pretzel on one of our first tests. So we replaced it and reinforced it.

 

[spork]

oh bugger. What an idiot! I screwed up my radians. Hang on. Let me get my thinking head on.

<edit> I multiplied 2 rpm by 60 instead of dividing...

Except for your ability to compute tip speed, you mig welders should think about teaching physics!

On the other hand, my welding is pretty friggin' horrible!


By the way, I don't recall the exact numbers right off, but our propeller had a diameter of around 16'. So a circumference of around 50'. We were probably somewhere around 120 rpm, so about 6000 ft/min tip speed (or 100 ft/sec), or around 70 mph.

 

[Screwdriver]

Thanks for that explanation, I think the penny has finally dropped with regards to the "cushion of air" I had assumed incorrectly. My "disobeying the laws of physics" (joke) above was while imagining Blackbird literally flying though still air in reference to your Cessna description. My only experience of aerodynamics is through riding a motorcycle at (very) high speed but I do appreciate it is a vastly complicated subject.

Thank you for shedding some light on the subject. It sort of has a knock on effect once you have a "seat of the pants" grasp of a phenomena, it helps form a mental picture for other similar issues. Every day is a school day.

 

[Screwdriver]

Except for your ability to compute tip speed, you mig welders should think about teaching physics!

On the other hand, my welding is pretty friggin' horrible!


By the way, I don't recall the exact numbers right off, but our propeller had a diameter of around 16'. So a circumference of around 50'. We were probably somewhere around 120 rpm, so about 6000 ft/min tip speed (or 100 ft/sec), or around 70 mph.

My Mig welding is also several orders of magnitude out. You must be thinking about some of the other chaps on here who are quite extraordinarily good in a very wide range of skills. This is a great forum for practical metalworking both for information and if required, design, manufacture. Any one of us would jump at the chance to be involved in a project like yours.

Oh and I went back over the video so I see where the "spork" moniker originates!

 

[PhillipM]

Don't look at me, I just weld scrap metal together and hope it doesn't fall apart bouncing off the terrain at 120mph, ain't winning any prizes on my weldin'

 

[Screwdriver]

You, my friend, understand Galilean invariance better than a disturbing number of physics professors! or (?)

Haha, thanks (I had to look that up!).

 

[spork]

My Mig welding is also several orders of magnitude out.

The best tip I got on welding was from Jamie Hyneman (Myth Buster). He told me to use my ears as much as my eyes. Big help, but I could still use a ton of practice.

Oh and I went back over the video so I see where the "spork" moniker originates!

You'll have to refresh my memory. Spork is just a name I picked many years ago since it's a sort of tool that's really pretty bad at everything. Although I sometimes tell people I was given that name when I killed a man in prison with a plastic spork (which made a huge mess and required a lot of patience on his part).

 

[Screwdriver]

But that bubble would have to be growing very rapidly for that tailwind to be able to continue pushing on it since we're going 10% faster than the wind. The reality is that the propeller simply does what a propeller will do when moving through the air at 1 m/s. Makes sense?

If I'm going downwind at 1.1X wind speed, the wind can't push me as a bluff body. I'll be experiencing a headwind of 0.1X wind speed.

Once the vehicle has exceeded the speed of the wind, I see how the propeller is now screwing itself into the air ahead of the prevailing wind. That area ahead is of course, also moving with the gigantic mass of the atmosphere in that region. The vehicle is therefore able to pull itself along at whatever thrust the prop can develop. I am still very curious as to what happens to the disturbed air behind the vehicle if you have any thoughts on that.

It must have dissipated more energy locally (in that one small area) than it would have done had the vehicle not been there (I might be wrong about that of course). But if I have this right, you're actually sucking energy out of the spinning globe, i.e. the wind would have been depositing a huge amount of energy trying to spin the earth, now you're stealing some of that to drive the earth backwards using the wheels.

 

[Screwdriver]

The best tip I got on welding was from Jamie Hyneman (Myth Buster). He told me to use my ears as much as my eyes. Big help, but I could still use a ton of practice.



You'll have to refresh my memory. Spork is just a name I picked many years ago since it's a sort of tool that's really pretty bad at everything. Although I sometimes tell people I was given that name when I killed a man in prison with a plastic spork (which made a huge mess and required a lot of patience on his part).

One of the professors in this video refers to "the guy pushing the treadmill model with a spork". A curiously pedantic observation I thought but typical of that level of genius.